Petrogenesis and Tectonic Evolution of the Kahnouj Ophiolitic Complex (SE Iran): Constraints from geochemistry and Sr-Nd-Pb isotope composition

Abstract

Based on new Sr-Nd-Pb isotope-compositional and lithogeochemical data combined with previously published data of the Kahnouj ophiolite, we propose a tectonic model for the Kahnouj ophiolitic complex. The Kahnouj ophiolitic complex is the largest ophiolite of the Makran zone and consists of isotropic and layered gabbros with ultramafic lenses at the bottom and sheeted dikes, and basalts and pelagic limestones on the top. A cyclic succession of isotropic and layered gabbros indicates a distinct differentiation trend of early clinopyroxene crystallization followed by hornblende. Crystal fractionation and partial melting processes are also inferred by Nd isotopic and whole-rock geochemical data (e.g., fractionation recorded by the Al2O3/TiO2 versus Ti/1000 diagram). Geochemically, the gabbros are subalkaline and show tholeiitic features. The εNd values of + 7 to + 7.6 and initial 87Sr/86Sr ratios of 0.70352 to 0.70377 indicate a mid-ocean ridge mantle source for the gabbroic rocks. The whole rock geochemistry of the layered and isotropic gabbros suggest that they formed in a back-arc basin setting and represent MOR- to supra-subduction type ophiolites in the Neotethys Ocean during Jurassic to Early Cretaceous (156 to 121 Ma).